In general, direct dye products last only 6-10 shampoos and are hence known as semi-permanent. However, many consumers want more permanent results, and therefore default to oxidative dye products that contain hydrogen peroxide or other oxidants. Providing vibrant blue-violet to blue colors is challenging, because many of the vibrant dyes are not sufficiently stable for permanent haircolor compositions, and those that are stable to hair dyeing conditions are not sufficiently vibrant. The direct dye compounds and the method described herein can be used in either direct dye or oxidation dye products.
The permanent alteration of the color of keratinous fibers, in particular human hair, by the application of hair dyes is well known. In order to provide the consumer with the shade, longevity, and the intensity of color desired, an oxidative coloring process involving complex chemical reactions is utilized. Permanent hair dyeing formulations typically comprise primary intermediates (also known as oxidative hair dye precursors or developers) and couplers (also known as color modifiers or secondary intermediates). These dye precursors are sufficiently small, polar and soluble to diffuse into the hair shaft where, once activated by an oxidizing agent under basic conditions, such as hydrogen peroxide, the primary intermediates react with other dye precursors, e.g., couplers, to form larger colored chromophores in the hair shaft. The chromophores formed in the hair shaft do not readily diffuse out of the hair during subsequent washing with water and/or detergents because they are bigger, less polar and soluble than dye precursors that diffused in.
Hair colorant products are typically sold in the form of kits containing a dye component (e.g., a dye solution) and an oxidizing component (e.g., a hydrogen peroxide solution). In use, the dye component is mixed with the oxidizing component and the resultant mixture is applied to hair. When the two components are mixed, oxidizing agents present in the oxidizing component begins to oxidize primary intermediates present in the dye component and the oxidized primary intermediates begin to react with couplers to form chromophores. Since coloring hair is one of the beauty routines, it is highly desirable that the dyeing process, excluding bleaching, be rather a physical process, which would allow it to be aligned with many other beauty routines such as applying lip color and facial touchups. The challenge is to still meet all of the other requirements of hair color (e.g., washfastness, resistance to perspiration, little or no bleeding of color from the hair when it is wet, evenness, etc.).
Many attempts have been made by the hair color industry to enhance the washfastness of direct dyes by either forming a covalent bond between chromophore and proteins inside hair or increasing the number of binding sites, typically cationic centers, on the chromophore. However, each attempt has its drawbacks. The approach through covalent bonding does not differentiate proteins in hair from skin. The approach through multiple binding sites on the dyes (i.e. multiple positive charges to interact with negative sites on hair, either by bonding several monocationic dyes together or by installing multiple cationic centers on a single chromophore) runs into the obstacles of uneven color due to uneven damage (negative charges) along the length of the hair fibers and reduced dye penetration into hair fibers because the dyes are typically at least twice as large as common oxidative dye precursors. An increase in the number of binding sites minimizes bleeding and color loss caused by rinsing by providing stronger hair-chromophore interactions. However, the same strong binding force to the cuticle also prevents the chromophores from penetrating deep into the cortex of hair, because it is difficult for dyes with multiple positive charges to diffuse through negatively charged networks of keratin proteins. Additionally, since polycationic dyes remain bound to the hair surface rather than penetrating into the fiber, it is difficult to produce dark shades, due to limited binding sites on the surface of hair.
In general, blue-violet to blue colors used for dyeing or hueing agents for materials such as plastics, fabrics, and hair are either fused polycyclics such as triarylmethanes, substituted phenoxazines and anthraquinones, carbocyclic azo dyes in which the acceptor half of the molecule is highly substituted with electron-withdrawing groups used to shift the color into the desired range, bis(azo) compounds that extend conjugation, or azo dyes that have a carbocyclic donor portion and a thiazolium or thiazole (substituted with electron-withdrawing groups such as cyano or nitro) acceptor portion. Considering the donor-acceptor approach of color design, the well-known tactics are to include powerful election-withdrawing groups in the acceptor end of the molecule, thus pulling electron density towards the acceptor portion, driving the color to higher absorbance wavelengths. Although it can give the desired color shift, it can decrease the stability of the dyes, particularly when the acceptor end is a heterocycle such as imidazolium or thiazolium.
Although these well-known and well-accepted aforementioned classes of dyes provide the blue-violet to blue colors desired, each of these classes has at least one flaw that makes it inappropriate for dyeing at alkaline pH in highly aqueous systems, such as in hair dye products or highly aqueous dyebaths. (More aqueous dyebaths are desired to decrease the environmental impact relative to dyebaths that use high levels of organic solvents.) The solubility, fastness properties, or stability of current materials may be insufficient for the applications. In cases in which solubility, resistance to removal by washing and perspiration, and stability are sufficient, the current materials do not have the required blue hue. Specifically in applications such as hair dyeing, this is an important outage.
Thiazolium and thiazole azo compounds in particular provide brilliant colors with high intensity. However, it is well-known that particularly for the vibrant and water soluble thiazolium dyes, stability is poor. Common, commercial imidazolium dyes (analogs of the thiazolium dyes) can give brilliant and stable orange to violet-red colors. However, there are no good examples of blue imidazolium dyes, most likely because using the common approach of adding electron-withdrawing groups to the acceptor (imidazolium) end of the molecule results in materials that are highly unstable, especially at alkaline pH.
Baumann and Dehnert described generating a violet imidazolium azo dye (in Chimia 15, 1961, 163-168) however they could not obtain blues, and Abbot, et al. (in J. Phys. Chem. J. Phys. Chem. A 2013, 117, 1853-1871) demonstrated the inherent instability of thiazolium compounds under basic conditions, making them unsuitable for applications for which neutral to alkaline pH is necessary.
Accordingly, there is a need for a direct dye compound with improved washfastness without the drawbacks previously described, and these imidazolium azo dyes meet all the criteria of color, stability, and fastness to be used as permanent hair dye compounds.